The present invention relates generally to the melt processing of high temperature metals. More specifically, it relates to methods by which the contamination of high temperature melts can be reduced, prevented and/or avoided.
It is known that in the processing of lower temperature melts of metals contamination from atmospheric oxidation or from impurities introduced into the melt from the melt crucible, or from dust particles is at an exemplary low level. Ordinary procedures and practices permit melting and casting to be accomplished without exceeding the acceptable levels of impurities in such metals. Metals, such as lead, zinc, tin, bismuth, as well as alloys such as brasses, bronzes, and the like, have been usefully and successfully processed through a melt phase without impairment of the solid product metal through the introduction of an excessive level of impurities or contaminants due to the processing. Such metals are melted at lower melting temperatures of the order of a hundred to a few hundred degrees. Heat can be delivered to such melts through their containing crucible and such heating generates very little vaporous or particulate matter.
For metals which melt at higher temperatures, and particularly above about 1000.degree. C., the techniques employed in the melting and the techniques for keeping the melt free from contamination, either from the atmosphere or from impurities, are of a different character.
In the first place, the means used for melting the metals which melt at much higher temperatures are different and, in the case of highly reactive metals such as titanium, may involve the use of a plasma flame or an electron beam or similar melting technique. The application of heat from such sources to the metal of the melt is directly onto the melt surface rather than through a crucible wall. In addition, because of the high reactivity of metals such as titanium, the metal must be protected from ordinary oxygen and nitrogen containing atmosphere. Further, because metal such as titanium is highly reactive with any crucible material, the metal is melted in a cold skull type of crucible in which a layer of solid titanium serves as the crucible for the liquid or molten titanium. Because of these unique circumstances, and because of the nature of the vaporous droplet and particulate material which is generated from the furnacing and melting of the high melting metal materials, special problems arise.
One such problem involves the deposit of vaporous and particulate material on the inside surfaces of enclosures provided to protect the molten metal from contact with ordinary atmospheres. The degree of vaporization and formation of particulate material is quite high for the high melting materials, at least partly because of the nature of heat delivery in the melting process itself. Heat is delivered from high temperature sources and is delivered at high intensity to a metal or melt surface. Plasma torch heat is delivered at temperatures in excess of 10000.degree. C., for example. Thus, it has been found that there is a substantial amount of vaporous and particulate material generated from the use of plasma flames directed downward onto the top of a melt in a cold hearth crucible. This vaporous and particulate matter deposits on all interior surfaces of the containing vessel. Also, where electron beam heating is employed, a substantial amount of sputtering, spattering, and dissipation of the solid and liquid material occurs to the degree that there is a formation on all of the internal surfaces of the enclosing vessel of a deposit of the vaporized, and/or particulate material.
As the use of the vessel continues, there is a tendency for the surface deposited material to flake and to drop off in a manner which permits contamination of the melt. Where a tank or vessel is employed in the melting or melt processing of a number of different alloys, one danger is that the deposit formed during processing of one alloy will flake off and fall into the melt of a different alloy thereby contaminating the later processed alloy.
Efforts are made to avoid such contamination and may involve cleaning of the furnace interior between runs. However, another problem occurs during a single run and cannot be cured by cleaning between runs. This problem is that the condensate on a vessel interior has a much higher concentration of the more volatile elements, such as aluminum, than the melt from which the vapor is generated. The aluminum content of a titanium alloy containing 6% aluminum originally may be as high as 50%. When this condensate forms during a single run and drops into the melt just prior to casting, substantial property disparities can result in the casting.
Another type of processing of metals having high melting temperatures is the rapid solidification plasma deposition. In this process particles of the metal to be melted are entrained in a carrier gas and are passed through a plasma flame. The production of fine particulate solids and of metal vapors during plasma spray processing of a powder through a melt phase is similar to that which occurs during the high temperature melting processes described above.
The portion of an enclosing vessel which is particularly susceptible to deposits which can enter the melt is the portion directly over the target mandrel. Flakes of deposit which fall from this "ceiling" portion of the vessel are directly over the mandrel or melt and can fall directly onto or into the mandrel or melt.